The present invention relates to a method for producing an article comprising at least a polyurethane layer, in which method a polyurethane reaction mixture is supplied under pressure to a spray nozzle, within the nozzle the reaction mixture is accelerated by passing it through one or more channels wherein the flow of reaction mixture achieves a minimum cross-sectional area measured perpendicular to the flow of the reaction mixture, the accelerated reaction mixture is sprayed, with a predetermined amount of kinetic energy, through a spray opening out of the spray nozzle onto a surface to produce said polyurethane layer and the reaction mixture is allowed to cure.
Such a method is disclosed in EP-B-0 303 305 and in EP-B-0 389 014. In the known method an elastomeric polyurethane skin layer showing a thickness larger than 0.3 mm, and preferably a thickness of between 0.5 and 2 mm, is produced by spraying a light stable polyurethane reaction mixture by means of an airless two-component spray nozzle in accordance with a predetermined spray pattern. The reaction mixture is more particularly sprayed out of the nozzle in the form of a film defining a hollow cone. Spraying of the reaction mixture is controlled so that this film falls apart into droplets having a medium volume diameter (M.V.D.), determined in accordance with ASTM E 799-81, of at least 100 μm, and preferably of at least 500 μm. The mould surface onto which the reaction mixture is sprayed may be the surface of a complex mould, designed for example to produce the skin of a dashboard and showing in particular narrow cavities and/or undercuts. According to EP-B-0 303 305 spraying the reaction mixture so that droplets of the above mentioned minimum average size are formed on a certain spray distance offers the advantage that the reaction mixture can be sprayed from varying spray distances without causing important differences in density and other physical properties such as the colour of the formed layer. This is of course especially important when the polyurethane layer has to be sprayed in complex moulds showing narrow cavities so that the spray distance cannot be kept constant.
A drawback of the known methods, and in particular of the spray nozzles used therein, is that in practice there is a lower limit for the minimum flow rate at which the reaction mixture can be sprayed. This lower limit is determined by the minimum dimensions of the nozzle which are feasible in practice and by the minimum flow rate of the reaction mixture required to obtain a stable spray pattern. The minimum dimensions of the spray nozzle are in particular determined by the minimum cross-sectional areas of the flow paths of the reaction mixture required to avoid clogging of the nozzle, for example when crystals are formed in the highly viscous reaction mixture. Especially the cross-sectional size of the channel or channels wherein the reaction mixture is accelerated is critical. These channels defined the minimum cross-sectional area of the flow path through the nozzle. As illustrated in Example 1 of EP-B-0 303 305 a nozzle showing 4 grooves or channels of 0.5×0.5 mm in the insert which is arranged in the nozzle to accelerate the reaction mixture and to create the required swirling motion of the reaction mixture in the cavity of the nozzle before spraying it out of the nozzle can be used to spray the reaction mixture at a flow rate of 25 g/sec. However, as illustrated in Example 2, for reducing the flow rate of the reaction mixture to 10 g/sec, only two grooves or channels can be provided in the insert, the dimensions of which have moreover been reduced to 0.3×0.3 mm. It is clear that the risk of clogging is increased in such narrow grooves and that the use of only two grooves may have a negative effect on the homogeneity of the spray pattern.
In order to be able to spray thinner polyurethane layers and/or polyurethane layers of a more uniform thickness, in particular also in narrow mould cavities of a complex mould wherein the reaction mixture is sprayed from a small spray distance, it should be possible to spray the reaction mixture at a sufficiently small flow rate. For this small flow rate, the spray pattern should still be sufficiently homogeneous and stable. Moreover, the small flow rate should be achieved without having to reduce the dimensions of the channel or channels wherein the reaction mixture is accelerated to such an extent that unacceptable clogging problems arise.